1. Field of the Invention
The invention relates to an electroacoustic capsule or transducer for an electroacoustic device. The transducer can operate either according to the electromagnetic, electrodynamic, electrostatic, or piezoelectric principle and can be embodied either as a sound emitter or a sound receiver.
2. Description of the Related Art
Such devices are comprised substantially of the actual electroacoustic transducer which is inserted into a so-called capsule which, in turn, is mounted in a device housing in which all required electronic components are also arranged.
Electroacoustic devices comprise at least one so-called electroacoustic capsule which is, in turn, either embodied as a sound source or a sound receiver. For the purpose of simplifying the language in the present description and claims, electroacoustic devices which comprise at least one capsule functioning as a sound receiver are referred to as microphones. Headsets are mentioned as being representative of electroacoustic devices with at least one electroacoustic capsule which is embodied as a sound source.
The two device groups however have one commonality: the acoustic properties of the devices are predetermined by the manufacturer during the course of the production process and are therefore unchangeable by the consumer. Expressed more simply, the device has an unchangeable sound characteristic.
For example, the acoustic properties of a microphone with an electrostatic capsule depend essentially on the spacing between the diaphragm and the electrode and on the design of the acoustic tuning elements of the capsule. When the geometric parameters between the movable electrode (the diaphragm), which is exposed to the sound field, and the stationary electrode are fixed and when also the acoustic tuning elements in the interior of the capsule (narrow channels, closed volumes, and only partially air-permeable areas) are calculated and mechanically realize, then the directivity pattern, the output level, and the frequency response characteristic are also fixed and unchangeable.
The capsule is therefore always configured with respect to the intended use, and it is generally not possible to employ an existing capsule in another housing or device without suffering great quality losses. This is true for sound receiving as well as sound emitting capsules.
This property requires a series of capsule developments, not to mention stocking expenses and providing different tools for their manufacture, which, in particular, in view of the currently conventional fast model changes, can become expensive very quickly.
The acoustic tuning of electroacoustic capsules, independent of whether they are manufactured as a sound receiver or sound emitter, must not be determined in series of experiments at random, but can be calculated within wide ranges. This calculation is based on the matching mathematical models for acoustics and electricity and is carried out based on the electroacoustic analogy principle. It is performed by means of so-called equivalent circuits. In this connection, narrow and long channels in the acoustic system correspond to a coil in the electric system, closed volumes in the acoustic system correspond to the capacitor in the electric system, and bores covered with porous or only partially air-permeable material in the acoustic system correspond to an ohmic resistance in the electric system. Accordingly, the acoustic side can be transferred into a circuit diagram which is dimensioned and tuned according to the general rules of electrical engineering in the desired way, and the result is then transferred back into the acoustic system.
By combining all three electroacoustic elements, it is thus possible to perform the desired tuning of the respective electroacoustic transducer. It has been shown that in particular narrow channels play an important role for an expedient tone color tuning of electroacoustic transducers. This is based on the fact that a narrow channel not only has an inductive impedance proportion but also a considerable large proportion of ohmic resistance. The generation of the latter can be traced back to flow losses in narrow channels.
Based on this knowledge, a so-called “friction pill” has been produced which has ohmic as well as inductive proportions with regard to its impedance and is described in AT 400 910 B. This patent document suggests to connect two plates, made of hard material and provided with small openings on their edges, by means of a screw at the center of the plates. By a targeted rotation of the plates relative to one another it is possible to affect the impedance of this configuration in the axial direction.
Another known possibility of changing the impedance resides in that the plates are not rotated relative to one another, but the spacing between the plates is changed by means of the central screw. The impedance change of the resulting so-called friction pill has an effect mainly on the sound of the microphone or the headset. This means that simultaneously not only the frequency response characteristic but also the directivity pattern of the microphone or the headset is changed. In any case, and independent of whether the tuning elements of the capsule can be changed during production or not, the acoustic tuning is carried out presently only once, i.e., before assembly of the capsule, and remains unchanged during the entire service life of the electroacoustic device. This is a condition which is only hesitantly accepted by the users of the microphones or the headsets.
Not only the sound characteristic of the electroacoustic device is decisive for its appropriate use. Its properties relative to the transmission quality are also important. They are determined primarily by the output level of the electroacoustic transducer.
Further relationships are the following. In addition to the described effect of an acoustic impedance pill (friction pill), the spacing between the electrode and the diaphragm affects the capsule capacitance and thus the output level of the capsule. The above described capsule, as a result of its mounting in a microphone housing, is connected electrically to the input of an amplifier provided within the microphone housing. By doing so, electroacoustic transmission properties of the microphone are determined significantly by both components. This means that the lowest as well as the highest sound pressures which can be transmitted without significant decrease of the transmission quality depend on the transmission properties of the microphone capsule and the microphone amplifier.
The lowest sound intensities which can still be transmitted are limited downwardly by the so-called background noise of the microphone. This is a thermal noise which occurs in all electronic devices. The strongest sound intensities still to be transmitted are limited by the limited power supply of the microphone amplifier because it is impossible for the output voltage of an amplifier to become greater than its supply voltage.
Development engineers in the electroacoustic field are always attempting to construct electroacoustic devices such that they can transmit very low volume as well as very high volume sound events without substantial quality losses. In order to configure a microphone capsule for even smaller sound pressures, it has to be configured such that it is as responsive as possible relative to sound pressure fluctuations. This means that its transmission factor should be as large as possible. This can be achieved with electrostatic sound receivers such that the spacing between the electrodes is as small as possible. On the other hand, in the case of very high sound pressures, the electric voltage at the input of the amplifier becomes so high that the output voltage of the amplifier, even for a sound pressure that is lower than before, reaches the level of the supply voltage of the amplifier as a natural amplification limit. This means that a compromise must be accepted with respect to the minimal and maximal sound pressures still to be transmitted, the so-called dynamic response.
However, when it is known that in a recording situation only very low volume sound events, for example, piano passages of a concert, or only very high volume sound events, for example, a percussion recording, are to be expected, the described disadvantages can be partially alleviated by placing the microphones in strategic places. This means that in the case of low volume sound sources the microphone is to be placed closer to the sound source and in the reverse situation of loud instruments the microphone is to be moved father away from the sound source. However, it is apparent that this can be realized only with difficulty and only in very rare situations.
Some microphone manufacturers alleviate this dilemma by mounting a so-called attenuator. A voltage divider between the capsule and the amplifier is switched on manually as needed so that for high volume sound events the amplifier does not receive a capsule signal that is too large. The attenuation of the microphone capsule signal is performed for electrostatic microphone transducers within the high-resistivity range, and this results in a series of circuit-technological difficulties. Primarily, for high-resistivity circuits suitable switches must be used. This means that only special and thus expensive switches can be used. Since the discussed example relates to a microphone capsule operating according to the electrostatic principle, which is represented as a capacitor in the electric circuit of the microphone, it is required to use so-called capacitive voltage dividers. They are realized with the aid of electric capacitors and make possible the desired signal attenuation within a broad range. However, unfortunately the total harmonic distortion (distortion of the output signal) increases audibly when a capacitive attenuator is used for such capsules. Therefore, such microphones are avoided for high-quality applications.